In order to cope with global water shortage and water contamination, a water treatment technology involving using a reverse osmosis membrane (RO membrane) has been attracting attention. Of such membranes, a reverse osmosis membrane using an aromatic polyamide that can remove even an ion component, such as salt, with a pore diameter of 1 nm or less has been most widely spread in seawater desalination plants.
In a water treatment involving utilizing a reverse osmosis membrane, a permeate flux and desalination performance (NaCl rejection rate) reduce with time owing to the adhesion of dirt to the reverse osmosis membrane. Accordingly, the permeate flux and the desalination performance needs to be restored by periodically washing the reverse osmosis membrane. A washing method desired for the washing of the reverse osmosis membrane is washing with an aqueous solution containing a hypochlorite or active chlorine having an oxidizing power because the washing is excellent in decomposing and removing properties for a protein component and the like, microbicidal property, and sterilizing property.
However, the reverse osmosis membrane using the aromatic polyamide has low resistance to chlorine having an oxidizing power, and hence a portion in contact with chlorine deteriorates. Accordingly, the membrane involves a problem in that its desalination performance extremely reduces after washing with oxidizing chlorine relative to that before the washing.
Reverse osmosis membranes using carbon nanotubes have also been proposed (Non Patent Literatures 1 to 3). However, each of the proposals is inferior in performance to a reverse osmosis membrane currently available on the market, and shows no dramatic improvement in chlorine resistance.